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Every time I teach this law, there are always a few students asking me why a net force is zero on a system moving at a constant velocity. This group of students takes physics for the 1st time. It is counter-intuitive to them since they believe a net force should be non-zero for a system to move at a constant velocity. I try to convince them this is an empirical phenomenon that cannot be fully explained, and they have to accept it is the way it is. Of course, it backfires, and students continue with "But why?"

I did come up with an analogy: Asking why about this law is like asking, "Why do like charges repel and unlike charges attract?" (These are all the fundamentals that are not fully explained and understood) It is the way it is in our nature! Sadly, I still can't convince them and even get blamed for not explaining physics clearly. I am frustrated and need help. Can anyone provide more physics scenarios or analogies so that I can totally convince my students? Thank you!

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  • $\begingroup$ I think the canonical way to do this is to throw something at them, and then ask them why they didn't expect it to abruptly stop moving toward them. Or invoke fun stuff like bowling balls or air hockey where there's naturally a lot of "sustain" so to speak. $\endgroup$
    – meltyness
    Sep 24, 2022 at 1:46
  • $\begingroup$ I think teachers shouldn't say that something cannot be fully explained, it cannot satisfy any student. My teacher in my younger times asked me where is the difference between motioning object and resting object, is there any middle state between them. In that way he tried to prove that these two phenomenons are the same. Also you can explain them that external forces cancel on laying rock on the ground, but from thew POV of person in the moving car the rock is in a state of motion. $\endgroup$
    – Aslan Monahov
    Sep 24, 2022 at 1:50
  • $\begingroup$ my answers here might help physics.stackexchange.com/questions/262917/… , physics.stackexchange.com/questions/44196/… $\endgroup$
    – anna v
    Sep 24, 2022 at 2:13
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    $\begingroup$ "It is counter-intuitive to them that external forces aren't supposed to cancel on a system in an equilibrium state."... this might be a typo, but the sentence as written doesn't seem to make sense. Hopefully that is not the source of confusion in your explanation. $\endgroup$
    – RC_23
    Sep 24, 2022 at 2:43
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    $\begingroup$ You can show this by changing reference inertial frames. In your frame u r at rest but someone moving at constant velocity wrt you, they are at rest in their frame. From here you can build the argument. Or you can talk about how the object must give its kinetic energy to another body to slow down, but since there are no other bodies in the example to give energy to, velocity must remain constant. $\endgroup$
    – GedankenExperimentalist
    Sep 24, 2022 at 23:01

4 Answers 4

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You might try giving them a lesson on the history of physics. Where did Newton's first law come from? Well, it came from a series of experiments performed by Galileo. In those experiments he let a ball roll down one ramp and then up another. Galileo noted that the ball would roll up to the same height on the second ramp, regardless of the angle of the two ramps. Galileo then imagined that if he could make the second ramp flatter and flatter the ball would never stop (but for friction) because it could never get back up to the same height. You can find lots of videos and articles on this particular subject by searching for them. The nice part about this one is that it should be relatively cheap to cook up a demo for it using steel ball bearings and some wooden ramps with v-shaped tracks in the top.

From there you could move on to the second example Newton probably had in mind when he formulated his laws: the Moon. Why doesn't the Moon crash into the Earth? Because if it needed a force to maintain it's sideways velocity, it most certainly would crash into the Earth very soon.

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  • $\begingroup$ My approach was different. I explained how it began with Aristotle's Law of Motion until Newton clarified his statement and then crafted the 1st Law. Thank you! Good point though! $\endgroup$
    – TBS500
    Sep 24, 2022 at 6:19
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Newton's first law is, unfortunately, not a trivial proposition. What it tells us is that there are inertial systems in which the motion of objects is, as my high school physics teacher put it, "as simple as it can be". The existence of such systems is an empirical observation, even though it is a non-trivial one on the surface of Earth which is obviously not an inertial system.

It is important to train students to accept empirical information as the guiding principle in physics. They have to understand that physics is not mathematics. It does not "derive" its results from some abstract axioms. Everything we do in physics starts and ends with observations and experiments.

The primary, I would even say primal observations that "things at rest tend to stay at rest" and "things in motion follow a straight line" have to be demonstrated experimentally beyond any doubt. That is what the air-table experiments are for. They are not just a sixty-second demonstration of the first law. They are one form of experimental evidence for the first law and ideally students should be allowed to make this experience hands on.

In other words: the work of the physics teacher does not start by introducing laws and formulae that the students have to learn to pass the exams. It starts by actually demonstrating physics hands-on and by giving students the confidence to trust their observations. The question "Why is this so?" has to be replaced by "I have seen with my own eyes that it is so!". Only if we see an air puck that moves in a straight line over and over again can we learn to trust that "nature likes straight lines". Only when the confidence in these observations is gained can the student be taught how to analyze those straight line motions and how to make good use of them for more complicated scenarios.

There is, of course, a higher level of theoretical mechanics that explains exactly "Why this is so.", but I doubt that you will be successful in teaching the majority of high school students about the least action principle, Noether's theorem and differential geometry. Maybe a few of your students will go to university and learn the modern perspective on Newton's laws, but they can only do that successfully if you instill the necessary trust in them that "If nature says so, then it is so.".

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  • $\begingroup$ Existence of inertial systems is important and can be postulated, but I doubt this is what Newton thought about the 1st Law. $\endgroup$
    – kludg
    Sep 24, 2022 at 6:51
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Objects slow down and come to a stop because of forces holding them back. For a smaller force, it takes longer to slow to a stop.

  • A car slows to a stop if you step on the brake.
  • A car takes longer to stop if you step on the brake gently.
  • If you don't step on the brake at all, there is still the wind holding you back. You can feel it if you stick your hand out the window.

Everything moving thing we see comes to a stop. There is always a force somewhere. Sometimes it is hard to find the force.

  • There isn't much wind pushing on a car at 1 mph. But there is some.
  • A car skidding on on ice skates eventually comes to a stop. Ice is slippery, but not perfectly slippery.

If you make the forces smaller, the slowing is less. Imagine you find all these forces and make them disappear. There would be no slowing at all. The car would keep going at the same speed forever. Sadly, we never get to see this happen.

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  • $\begingroup$ Thanks, but I think it doesn't answer the question since you are talking about Newton's 2nd Law. $\endgroup$
    – TBS500
    Sep 24, 2022 at 6:32
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    $\begingroup$ Newton's first law is about how things in uniform motion don't stop, so understanding it is equivalent to creating an abstract notion of frictionlessness. To go to that notion, you have to imagine situations with less and less friction, until (conceptually) there is none at all. Then you can bring up Newton's first law and its consequences. I use air hockey tables with the air turned off, then on, to build the intuition of less-and-less friction. $\endgroup$
    – Peter Leopold
    Sep 24, 2022 at 12:45
  • $\begingroup$ @TBS500 A lot of people aren't aware of all the various forces that act on everyday objects, grinding them to a halt relative to the surface of the Earth. They imagine their hand pushing a paper across a table as the only force, or their engine pushing the car forward, and say, "clearly this object with net-nonzero force applied to it keeps a constant speed, while the object with zero force slows down", making Newton's first law unintuitive. You have to then explain those other forces. So yes, this really does answer the question, and better than you realize. $\endgroup$
    – hegel5000
    Sep 30, 2022 at 19:26
  • $\begingroup$ @TBS500 to put my comment another way: this answer appears to be about Newton's 2nd law because Newton's 1st law doesn't make any sense in isolation of the 2nd. $\endgroup$
    – hegel5000
    Sep 30, 2022 at 20:29
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You could point out that planets keep going even though the force of gravity only pulls them in. There is no force pushing the planets forward along their direction of motion.

The following might be too abstract. If you move with an object that is moving at constant velocity then, relative to you, it's not moving. An object at rest will stay at rest. Nothing will change. It will not spontaneously accelerate from rest. Therefore it won't accelerate in any other constant velocity frame of reference. Again, nothing will change. Because there is no force acting.

You could also show them these demonstrations where people jump on moving trampolines. Note how they keep moving forward despite no force acting on them.

https://www.youtube.com/shorts/GIMNS0YiHzY

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